Metadata

Metadata

Nova presents configuration information to instances it starts via a mechanism called metadata. These mechanisms are widely used via helpers such as cloud-init to specify things like the root password the instance should use.

This metadata is made available via either a config drive or the metadata service and can be somewhat customised by the user using the user data feature. This guide provides an overview of these features along with a summary of the types of metadata available.

Types of metadata

There are three separate groups of users who need to be able to specify metadata for an instance.

User provided data

The user who booted the instance can pass metadata to the instance in several ways. For authentication keypairs, the keypairs functionality of the nova API can be used to upload a key and then specify that key during the nova boot API request. For less structured data, a small opaque blob of data may be passed via the user data feature of the nova API. Examples of such unstructured data would be the puppet role that the instance should use, or the HTTP address of a server from which to fetch post-boot configuration information.

Nova provided data

Nova itself needs to pass information to the instance via its internal implementation of the metadata system. Such information includes the requested hostname for the instance and the availability zone the instance is in. This happens by default and requires no configuration by the user or deployer.

Nova provides both an OpenStack metadata API and an EC2-compatible API. Both the OpenStack metadata and EC2-compatible APIs are versioned by date. These are described later.

Deployer provided data

A deployer of OpenStack may need to pass data to an instance. It is also possible that this data is not known to the user starting the instance. An example might be a cryptographic token to be used to register the instance with Active Directory post boot – the user starting the instance should not have access to Active Directory to create this token, but the nova deployment might have permissions to generate the token on the user’s behalf. This is possible using the vendordata feature, which must be configured by your cloud operator.

The metadata service

Note

This section provides end user information about the metadata service. For deployment information about the metadata service, refer to the admin guide.

The metadata service provides a way for instances to retrieve instance-specific data via a REST API. Instances access this service at 169.254.169.254 and all types of metadata, be it user-, nova- or vendor-provided, can be accessed via this service.

Using the metadata service

To retrieve a list of supported versions for the OpenStack metadata API, make a GET request to http://169.254.169.254/openstack, which will return a list of directories:

$ curl http://169.254.169.254/openstack
2012-08-10
2013-04-04
2013-10-17
2015-10-15
2016-06-30
2016-10-06
2017-02-22
2018-08-27
latest

Refer to OpenStack format metadata for information on the contents and structure of these directories.

To list supported versions for the EC2-compatible metadata API, make a GET request to http://169.254.169.254, which will, once again, return a list of directories:

$ curl http://169.254.169.254
1.0
2007-01-19
2007-03-01
2007-08-29
2007-10-10
2007-12-15
2008-02-01
2008-09-01
2009-04-04
latest

Refer to EC2-compatible metadata for information on the contents and structure of these directories.

Config drives

Note

This section provides end user information about config drives. For deployment information about the config drive feature, refer to the admin guide.

Config drives are special drives that are attached to an instance when it boots. The instance can mount this drive and read files from it to get information that is normally available through the metadata service.

One use case for using the config drive is to pass a networking configuration when you do not use DHCP to assign IP addresses to instances. For example, you might pass the IP address configuration for the instance through the config drive, which the instance can mount and access before you configure the network settings for the instance.

Using the config drive

To enable the config drive for an instance, pass the --config-drive true parameter to the openstack server create command.

The following example enables the config drive and passes a user data file and two key/value metadata pairs, all of which are accessible from the config drive:

$ openstack server create --config-drive true --image my-image-name \
    --flavor 1 --key-name mykey --user-data ./my-user-data.txt \
    --property role=webservers --property essential=false MYINSTANCE

Note

The Compute service can be configured to always create a config drive. For more information, refer to the admin guide.

If your guest operating system supports accessing disk by label, you can mount the config drive as the /dev/disk/by-label/configurationDriveVolumeLabel device. In the following example, the config drive has the config-2 volume label:

# mkdir -p /mnt/config
# mount /dev/disk/by-label/config-2 /mnt/config

If your guest operating system does not use udev, the /dev/disk/by-label directory is not present. You can use the blkid command to identify the block device that corresponds to the config drive. For example:

# blkid -t LABEL="config-2" -odevice
/dev/vdb

Once identified, you can mount the device:

# mkdir -p /mnt/config
# mount /dev/vdb /mnt/config

Once mounted, you can examine the contents of the config drive:

$ cd /mnt/config
$ find . -maxdepth 2
.
./ec2
./ec2/2009-04-04
./ec2/latest
./openstack
./openstack/2012-08-10
./openstack/2013-04-04
./openstack/2013-10-17
./openstack/2015-10-15
./openstack/2016-06-30
./openstack/2016-10-06
./openstack/2017-02-22
./openstack/latest

The files that appear on the config drive depend on the arguments that you pass to the openstack server create command. The format of this directory is the same as that provided by the metadata service, with the exception that the EC2-compatible metadata is now located in the ec2 directory instead of the root (/) directory. Refer to the OpenStack format metadata and EC2-compatible metadata sections for information about the format of the files and subdirectories within these directories.

Nova metadata

As noted previously, nova provides its metadata in two formats: OpenStack format and EC2-compatible format.

OpenStack format metadata

Changed in version 12.0.0: Support for network metadata was added in the Liberty release.

Metadata from the OpenStack API is distributed in JSON format. There are two files provided for each version: meta_data.json and network_data.json. The meta_data.json file contains nova-specific information, while the network_data.json file contains information retrieved from neutron. For example:

$ curl http://169.254.169.254/openstack/2018-08-27/meta_data.json
{
   "random_seed": "yu5ZnkqF2CqnDZVAfZgarGLoFubhcK5wHG4fcNfVZEtie/bTV8k2dDXK\
                   C7krP2cjp9A7g9LIWe5+WSaZ3zpvQ03hp/4mMNy9V1U/mnRMZyQ3W4Fn\
                   Nex7UP/0Smjb9rVzfUb2HrVUCN61Yo4jHySTd7UeEasF0nxBrx6NFY6e\
                   KRoELGPPr1S6+ZDcDT1Sp7pRoHqwVbzyJZc80ICndqxGkZOuvwDgVKZD\
                   B6O3kFSLuqOfNRaL8y79gJizw/MHI7YjOxtPMr6g0upIBHFl8Vt1VKjR\
                   s3zB+c3WkC6JsopjcToHeR4tPK0RtdIp6G2Bbls5cblQUAc/zG0a8BAm\
                   p6Pream9XRpaQBDk4iXtjIn8Bf56SCANOFfeI5BgBeTwfdDGoM0Ptml6\
                   BJQiyFtc3APfXVVswrCq2SuJop+spgrpiKXOzXvve+gEWVhyfbigI52e\
                   l1VyMoyZ7/pbdnX0LCGHOdAU8KRnBoo99ZOErv+p7sROEIN4Yywq/U/C\
                   xXtQ5BNCtae389+3yT5ZCV7fYzLYChgDMJSZ9ds9fDFIWKmsRu3N+wUg\
                   eL4klxAjRgzQ7MMlap5kppnIYRxXVy0a5j1qOaBAzJB5LLJ7r3/Om38x\
                   Z4+XGWjqd6KbSwhUVs1aqzxpep1Sp3nTurQCuYjgMchjslt0O5oJjh5Z\
                   hbCZT3YUc8M=\n",
   "uuid": "d8e02d56-2648-49a3-bf97-6be8f1204f38",
   "availability_zone": "nova",
   "keys": [
       {
         "data": "ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAAAgQDYVEprvtYJXVOBN0XNKV\
                  VRNCRX6BlnNbI+USLGais1sUWPwtSg7z9K9vhbYAPUZcq8c/s5S9dg5vTH\
                  bsiyPCIDOKyeHba4MUJq8Oh5b2i71/3BISpyxTBH/uZDHdslW2a+SrPDCe\
                  uMMoss9NFhBdKtDkdG9zyi0ibmCP6yMdEX8Q== Generated by Nova\n",
         "type": "ssh",
         "name": "mykey"
       }
   ],
   "hostname": "test.novalocal",
   "launch_index": 0,
   "meta": {
      "priority": "low",
      "role": "webserver"
   },
   "devices": [
       {
         "type": "nic",
         "bus": "pci",
         "address": "0000:00:02.0",
         "mac": "00:11:22:33:44:55",
         "tags": ["trusted"]
       },
       {
         "type": "disk",
         "bus": "ide",
         "address": "0:0",
         "serial": "disk-vol-2352423",
         "path": "/dev/sda",
         "tags": ["baz"]
       }
   ],
   "project_id": "f7ac731cc11f40efbc03a9f9e1d1d21f",
   "public_keys": {
       "mykey": "ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAAAgQDYVEprvtYJXVOBN0XNKV\
                 VRNCRX6BlnNbI+USLGais1sUWPwtSg7z9K9vhbYAPUZcq8c/s5S9dg5vTH\
                 bsiyPCIDOKyeHba4MUJq8Oh5b2i71/3BISpyxTBH/uZDHdslW2a+SrPDCe\
                 uMMoss9NFhBdKtDkdG9zyi0ibmCP6yMdEX8Q== Generated by Nova\n"
   },
   "name": "test"
}
$ curl http://169.254.169.254/openstack/2018-08-27/network_data.json
{
    "links": [
        {
            "ethernet_mac_address": "fa:16:3e:9c:bf:3d",
            "id": "tapcd9f6d46-4a",
            "mtu": null,
            "type": "bridge",
            "vif_id": "cd9f6d46-4a3a-43ab-a466-994af9db96fc"
        }
    ],
    "networks": [
        {
            "id": "network0",
            "link": "tapcd9f6d46-4a",
            "network_id": "99e88329-f20d-4741-9593-25bf07847b16",
            "type": "ipv4_dhcp"
        }
    ],
    "services": [
        {
            "address": "8.8.8.8",
            "type": "dns"
        }
    ]
}

EC2-compatible metadata

The EC2-compatible API is compatible with version 2009-04-04 of the Amazon EC2 metadata service This means that virtual machine images designed for EC2 will work properly with OpenStack.

The EC2 API exposes a separate URL for each metadata element. Retrieve a listing of these elements by making a GET query to http://169.254.169.254/2009-04-04/meta-data/. For example:

$ curl http://169.254.169.254/2009-04-04/meta-data/
ami-id
ami-launch-index
ami-manifest-path
block-device-mapping/
hostname
instance-action
instance-id
instance-type
kernel-id
local-hostname
local-ipv4
placement/
public-hostname
public-ipv4
public-keys/
ramdisk-id
reservation-id
security-groups
$ curl http://169.254.169.254/2009-04-04/meta-data/block-device-mapping/
ami
$ curl http://169.254.169.254/2009-04-04/meta-data/placement/
availability-zone
$ curl http://169.254.169.254/2009-04-04/meta-data/public-keys/
0=mykey

Instances can retrieve the public SSH key (identified by keypair name when a user requests a new instance) by making a GET request to http://169.254.169.254/2009-04-04/meta-data/public-keys/0/openssh-key:

$ curl http://169.254.169.254/2009-04-04/meta-data/public-keys/0/openssh-key
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAAAgQDYVEprvtYJXVOBN0XNKVVRNCRX6BlnNbI+US\
LGais1sUWPwtSg7z9K9vhbYAPUZcq8c/s5S9dg5vTHbsiyPCIDOKyeHba4MUJq8Oh5b2i71/3B\
ISpyxTBH/uZDHdslW2a+SrPDCeuMMoss9NFhBdKtDkdG9zyi0ibmCP6yMdEX8Q== Generated\
by Nova

User data

User data is a blob of data that the user can specify when they launch an instance. The instance can access this data through the metadata service or config drive. Commonly used to pass a shell script that the instance runs on boot.

For example, one application that uses user data is the cloud-init system, which is an open-source package from Ubuntu that is available on various Linux distributions and which handles early initialization of a cloud instance.

You can place user data in a local file and pass it through the --user-data <user-data-file> parameter at instance creation.

$ openstack server create --image ubuntu-cloudimage --flavor 1 \
    --user-data mydata.file VM_INSTANCE

Note

The provided user data must be base64 encoded and is restricted to 65535 bytes.

Once booted, you can access this data from the instance using either the metadata service or the config drive. To access it via the metadata service, make a GET request to either http://169.254.169.254/openstack/{version}/user_data (OpenStack API) or http://169.254.169.254/{version}/user-data (EC2-compatible API). For example:

$ curl http://169.254.169.254/openstack/2018-08-27/user_data
#!/bin/bash
echo 'Extra user data here'

Vendordata

Note

This section provides end user information about the vendordata feature. For deployment information about this feature, refer to the admin guide.

Changed in version 14.0.0: Support for dynamic vendor data was added in the Newton release.

Where configured, instances can retrieve vendor-specific data from the metadata service or config drive. To access it via the metadata service, make a GET request to either http://169.254.169.254/openstack/{version}/vendor_data.json or http://169.254.169.254/openstack/{version}/vendor_data2.json, depending on the deployment. For example:

$ curl http://169.254.169.254/openstack/2018-08-27/vendor_data2.json
{
    "testing": {
        "value1": 1,
        "value2": 2,
        "value3": "three"
    }
}

Note

The presence and contents of this file will vary from deployment to deployment.

General guidelines

  • Do not rely on the presence of the EC2 metadata in the metadata API or config drive, because this content might be removed in a future release. For example, do not rely on files in the ec2 directory.

  • When you create images that access metadata service or config drive data and multiple directories are under the openstack directory, always select the highest API version by date that your consumer supports. For example, if your guest image supports the 2012-03-05, 2012-08-05, and 2013-04-13 versions, try 2013-04-13 first and fall back to a previous version if 2013-04-13 is not present.

Creative Commons Attribution 3.0 License

Except where otherwise noted, this document is licensed under Creative Commons Attribution 3.0 License. See all OpenStack Legal Documents.